This invention relates to a method and apparatus for testing the operability of the overload protection device of an electric induction motor.
Various electrical appliances employ electric motors provided with overload protection devices to protect the motor from overheating. A typical protective arrangement is a thermally responsive switching device with bimetal actuated contacts in series with the motor windings. The device, which is responsive to heat generated in the windings as well as heat generated by motor current through the bimetal device, opens the contacts when the bimetal temperature exceeds a predetermined temperature limit. During the manufacture of appliances equipped with such motors it is desirable to test the operability of the load protection device. When the motor output shaft is accessible, such testing can be accomplished simply by energizing the motor in normal fashion with a mechanical overload applied to the shaft. Under such conditions the bimetal temperature will rise quickly to the temperature limit and the protection device will trip if it is operating properly. However, in many applications the shaft is inaccessible. For example, in the manufacture of refrigerators the compressor motor is part of a sealed refrigerant system. In such situations it is necessary to create an overload condition by supplying electric current to the motor in such a way that the motor will remain stalled but will consume enough energy to trip the overload device. In order to minimize manufacturing costs a further requirement is that the current be sufficient to cause the device to trip quickly to limit the time required for testing
An AC power signal cannot be used for testing because at voltages low enough to keep the motor in its stalled condition, the power consumed is insufficient to trip the device. A DC power signal will heat the device quickly enough, but the back emf created by the windings as the contacts of the protection device begin to open, causes arcing which tends to weld the contacts closed. Half-wave rectified voltage can be used but the time required to generate sufficient heat to trip the device is longer than desirable. For example, with typical refrigerator compressor motors energized by a half-wave rectified line voltage signal, the time required is on the order of 30 seconds, which adds costy delay to the manufacturing process.
In view of the aforementioned shortcomings of the prior art it would be desirable to provide a method of testing the operability of the thermal overload device for induction motors which does not require accessibility to the motor shaft, which is fast, preferably less than 10 seconds, and which does not damage the device.
It is therefore an object of the present invention to provide a method of and apparatus for testing the operability of a thermal overload device for induction motors which applies a powe signal to the motor which keeps the motor in its stalled condition, trips the overload device in less than 10 seconds when the device is operating normally and which prevents the contacts from being welded during the test process.